This just reminds me of all those fake pre-release Brawl rosters filled with candidates such as Waluigi, Birdo, Hammer bro, Boo and every other unlikely random Mario kart char or Items, along with other random enemies or sidekicks from franchises you could possibly think of.
That one of those examples actually made it into Ultimate is just surreal to me, yet awesome nonetheless. This is proof yet again that we can't predict or outline Sakurai's inner/creative thought process.
If I had to guess how this came to pass, I would say it was probably one or more of the developers fooling around too much with Piranha Plant, which was most likely gonna be an Assist Trophy, but then finding/adding numerous attacks and mannerisms to this otherwise simple enemy and then being like; "Well shoot, we've gone this far, might as well make it a full fledged character".
I'm just wondering what this means for the future, are we gonna see a rising demand for Hammer bro or other popular enemies next? When are we gonna get the Deku baba echo/skin Sakurai?!